In conventional techniques for analyzing the composition of gas streams a gas sample is typically placed in an instrument such as a gas chromatograph or an optical or mass spectrometer which is outside of the stream. However, these instruments do not permit sensitive in-stream, real-time detection and analysis of gas samples. For example, it is very desirable to monitor the water content in the high-pressure helium coolant of a high-temperature gas reactor. Presently, this is accomplished by passing the helium sample over a cooled mirror which condenses the water. The presence of condensation is indicated by a change in reflection of a beam of light directed at the mirror. This technique exhibits a number of disadvantages, such as a long response time and complex instrument design. Further, it is often desirable to measure the presence and amount of numerous other trace elements and compounds, for example carbon, oxygen, sulfur, nitrogen, phosphorus, chlorine and fluorine, in high-pressure gas samples to which the mirror-condensation approach is not suited.
Trace element detection can be successfully performed at atmospheric and subatmospheric pressures using emission spectra obtained from microwave and direct current discharges. However, these techniques are not suitable for use in high pressure gas samples such as are found in the high temperature gas reactor.